1. Field of the Invention
The present invention deals with the field of refrigeration and, more particularly to cooling cryogens to a temperature below their triple point for an extended period.
2. Description of Prior Art
Prior solid cryogen coolers which have been used to cool sensors in space have operated by filling a thermally conducting matrix with liquid then freezing it either by means of external refrigeration or exposing it to a vacuum and which produces cooling, first by evaporation of some of the liquid and then by sublimation of the solid. The two techniques in which the forming of solid cryogens has been achieved circulate a coolant through a specially provided loop attached to a tank. In the first approach, room-temperature gas is introduced into the tank and subsequently liquified and frozen in a continuous manner, while in the second, the tank is filled with liquid which is subsequently frozen by the circulated coolant. The coolant loop is also used for periodic cooling of the solid during vehicle integration and checkout in order to maintain the cryogens in a no-loss (nonvented) condition. Liquid nitrogen can be used as the coolant for many of the cryogens, while liquid helium is required for the lower temperature cryogens such as neon and hydrogen. As the solid cryogen sublimes, it will recede from areas of maximum heat flux such as the tank walls, the support attachment points, and the thermal connection to the sensor. In order to achieve near isothermal conditions for the instruments, a heat exchanger is utilized inside the tank. Depending upon the requirements, extended fins of foam metal may be used in effectively creating an isothermal heat sink of the combined tank/cryogen/heat-exchanger system.
These solid cryogen coolers only provide cooling as long as there is solid cryogen in the matrix and thus have to be warmed up to be refilled.
The present invention eliminates the problems mentioned above by providing a cooler which cools by sublimation of a cryogen for an extended period of time. Other advantages of the present invention over the prior art will also be rendered evident.